Corrosion-resistant surface coating for use in the casting of aluminum and aluminum alloys

ABSTRACT

To prevent the corrosion of the surfaces contacting molten aluminum and aluminum alloy when such molten materials are being cast, a surface layer of boron impregnated ferrous materials is provided on the contacting surfaces of the containers and other conveying members used in the casting of the molten aluminum and its alloys.

United States Patent Kusunoki et al.

[451 Aug. 1, 1972 1541 CORROSION-RESISTANT SURFACE COATING FOR USE INTHE CASTING OF ALUMINUM AND ALUMINUM ALLOYS Inventors: KaneyoshiKmunoki; Tatsuml Assignees:

Filed:

Appl. No.:

Asakura;Masah1roNakakawa,allof Toyota-shi; Noboru Komatsu; Tohru Aral,both of Nagoya-shi, all of Japan Toyota .Ildosha Kogyo Kabushiki Kaisha,Toyota-shi, Aichi-ken; Kabmhiki Kaisha Toyota Chuo Kenltyusho,Nagoya-shi, Aichi-ken, Japan Nov. 13, 1969 Foreign Application Prioritypm April 15, 1969 Japan ..44/28689 US. Cl

Int. Cl.

....... ..l64/l38, 117/DIG. 10, 164/72,

...................... ..B22c l/00, B22c 3/00 Field of Search..l48/3l.5; 164/72, 138;

1l7/D1G. 10, 5.3; l06/38.9; 249/116 REDUCTION IN THlCKNESSlmm) PrimaryExaminer-J.

Spencer Overholser Assistant Examiner-V. K. Rising Attorney-McGlew andToren ABSTRACT To prevent the corrosion of the surfaces contactingmolten aluminum and aluminum alloy when such molten materials are beingcast, a surface layer of boron impregnated ferrous materials is providedon the contacting surfaces of the containers and other conveying membersused in the casting of the molten aluminum and its alloys.

llClaimZDrawingflgures ORON IMPREGNATED DURATION OF IMMERSION (nouns) muou'eu ALUMINUM ALLOYS PATENIEU 1'97? 3,680,626

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, "301w KOHRTSu TON'RU HRnl BY 5 M ATTORNEY CORROSION-RESISTANT SURFACECOATING FOR USE IN THE CASTING OF ALUMINUM AND ALUMINUM ALLOYS SUMMARYOF THE INVENTION The present invention is directed to the prevention ofcorrosion in the contacting surfaces of containers and other membersemployed in casting and forming ingots of aluminum and aluminum alloysand also to the prevention of the mixing of impurities into the aluminumand its alloys during the casting operation. Moreover, the invention isdirected to a boron impregnated ferrous material surface coating on themembers in contact with the aluminum and its alloys during the castingoperation.

Generally, pure aluminum is likely to combine with other elements and asa result to form various compounds. Similarly, aluminum alloys which maycontain quantities of copper, silicon, magnesia, nickel, manganese, andthe like, for the purpose of obtaining mechanical strength or forachieving other properties suitable for casting or working, does notexhibit the desired properties if impurities are mixed with the alloyelements. Moreover, the materials used in forming the containers andother members for casting pure aluminum and aluminum alloys and forforming ingots of the cast material must be such so as to prevent themixing of impurities into the molten aluminum and its alloys and also toprevent corrosion of the surfaces of the vessels and other memberscontacting the molten metal.

To prevent corrosion and the mixing of impurities into the molten metal,materials having stable properties at high temperatures, for instance,refractory oxides such as kaolin, quartz, magnesia, lime and others, orcarbonaceous materials such as graphite, are commonly used to line thecontainers and other members employed in the casting and moldingoperations. However, such surface linings are difficult to form and areeasily damaged and displaced. Moreover, in the interest of improving theproductivity and the strength of the cast products, continuous casting,low pressure casting and die casting operations have been used. In suchoperations, other conveying members of complicated shapes are immersedin the molten metal and molten metal is moved to other locations throughtransfer members or tubes. It is difficult and impractical to effect thesurface coating on such other members or tubes employing the prior artsurface coatings mentioned above.

Therefore, it is the primary object of the in vention to provide acorrosion-resistant surface layer for the containers and other membersused in the casting of aluminum and aluminum alloys and in molding'thealuminum and its alloys into ingots and the like, Further, it is theintention of the present invention to provide a surface coating which iseasily applied even to the contacting surfaces of containers or othermembers of complicated shapes or to the interior surfaces of tubularmembers. By employing the surface coating of the present invention, theinclusion of impurities into the aluminum and aluminum alloys being castcan be prevented and the life of the containers and other members beingused can be prolonged.

Therefore, in accordance with the present invention, at least thecontacting surfaces of containers and other members employed in thecasting and molding ofaluminum and aluminum alloys is provided with aboron impregnated ferrous material layer. In addition, the presentinvention encompasses the application of boron impregnated layers on thesurfaces of containers and other members used in casting and moldingaluminum and aluminum alloys, which are formed of carbon steel, castiron, cast steel, and. special purpose steels such as stainless steel.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a diagram illustrating the corrosion-resistant characteristicsof two samples of a stainless steel, one with and one without a boronimpregnated surface layer, immersed in molten aluminum alloy; and

FIG. 2 is a sectional view of a container used in casting aluminumhaving a surface coating in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 2, one exampleofa container 10 is shown which is used in the casting and molding ofaluminum and aluminum alloys. A surface layer 2 composed ofboron-impregnated ferrous materials is formed on the surface of thecontainer which contacts the molten aluminum and aluminum alloy.

It is well known to impregnate boron into structural steel and toolsteels for obtaining increased hardness characteristics on the surfacesof wear-resisting articles and for machine tools. Various impregnatingprocesses have been used including gas, solid, and liquid methods, andin certain instances the liquid methods may include an electrolyticoperation. In the casting and molding operation, variouscontainers andmembers are used, such as smelting crucibles, ladles, gutters, pipes,molds and the like. Such containers and members are made from ferrousmaterials, such as chrominum stainless steel and 18-8 stainless steel,and, in accordance with the present invention, the entire extent of suchcontainers or members or at least the surfaces which come in contactwith the molten aluminum and its alloys are impregnated with boron.

- As exemplary of the advantages obtained in corrosion-resistance byvirtue of the present invention, the

following examples are provided in which stainless steel samples weretested, certain of which were provided. with a boron impregnated surfacecoating.

EXAMPLE I ing of 90 percent by weight of borax and percent by weight ofzirconium oxide maintained at 950C, and at the same time it waselectrolyzed for 2 hours at a current'density of 0.5 A/crn' with thetest sample acting as the cathode. In this procedure a boron impregnatedlayer of about 40 microns in depth was formed on the surface of thesample. The test samples, one with and one without the boron impregnatedsurface coating, were immersed into a molten JIS AC4B aluminum alloy ofabout 2kg maintained at 700C. The depth of the corrosion on the testpieces was measured and is set forth in FIG. 1. The test sample coatedwith the boron impregnatedlayer corroded only 0.1mm in depth after aperiod of 30 hours,however, the untreated test sample corroded to adepth of 0.8mm in about 5 hours and the corrosion reached a depth of1.7mm after 30 hours.

EXAMPLE n In this example, a test sample of stainless steel having thesame dimensions and similarly impregnated with boron as in Example I,and an untreated test sample of HS FC cast iron having the dimensions of20 mm in diameter and 30mm in length were immersed into about 3kg ofmolten aluminum maintained at 750C. From past experience it has beendetermined that of the various ferrous metals, cast iron has the highestresistance to corrosion in molten aluminum. However, the resultsobtained in this test indicated that the sample of cast iron hadcorroded to a depth of about 2mm after 20 hours immersion, while theboron impregnated sample of stainless steel had corroded only to a depthof 0.1mm. Since the time period in which the molten aluminum or itsalloy are in contact with the surfaces of the containers or members inthe casting or molding operation is much less than the time periodsinvolved in the test examples, the amount of impurities mixed into theingots or cast products can be ignored. The useful life time of thecontainers and other members having the surface layer of boronimpregnated ferrous materials is prolonged and any inclusion ofimpurities into the aluminum or aluminum alloys will be decreased.-

In view of the various methods available forefl'ecting boronimpregnation, such as gas, liquid and solid methods, themost effectiveand efficient method of impr'egnation can be selected based on the shapeof the articles to be coated. Further, mixing of boron into aluminum andaluminum alloys is preferable since it tends to improve the quality ofthe castings because the addition of boron to aluminum and its alloysserves to refine the metallurgical structure.

We claim:

1. Method of corrosion and preventing the absorption of impurities inthe, manufacture of aluminum castings subject to corrosion andabsorption of impurities therein, which comprises:

a. heating the aluminum eating material and bringing the aluminummaterial to a molten state,

b. preparing a ferrous receptacle for receiving the molten aluminumcasting material by coating theinnersurfaceofsaidreceptaclewithathinlayerof boron metal,

c. placing the molten aluminum casting material into the ferrousreceptacle whose inner surface has n coa 'th boron metal, and owinguminum casting to cool, separating and recovering the cooled aluminumcasting from the receptacle.

2. Method according to claim 1, wherein said receptacle is a mold.

3. Method according to claim 1, wherein said receptacle is selected fromstainless steel, chromium stainless steel, and cast iron.

4. Method according to claim l, whereinthe boron metal'is coated on saidreceptacle to about 40 microns in depth.

5. Method according to claim 1, wherein the boron metal is coated onsaid receptacle at a temperature of about 950C.

' 6. Method according to claim 1, wherein said receptacle consists ofstainless steel.

7. Method according to claim 1, wherein said receptacle consists of castiron.

8. Method according to claim 1, wherein said coating of boron metal iseffected by electrolysis.

9. Method according to claim 8, wherein said electrolysis is carried outin a molten salt bath comprising an alkali metal borate.

10. Method according to claim 9, wherein said metal borate is sodiumborate.

11. Method according to claim 10, wherein the receptacle is prepared bydipping in a molten salt bath consisting of (i) about percent by weightof borax and (ii) about 10 percent by weight of zirconium oxide,maintaining the bath at a temperature of about 950C., electrolyzing thebath for about two hours at a current density of about 0.5 A/cm', thereceptacle being the cathode.

2. Method according to claim 1, wherein said receptacle is a mold. 3.Method according to claim 1, wherein said receptacle is selected fromstainless steel, chromium stainless steel, and cast iron.
 4. Methodaccording to claim 1, wherein the boron metal is coated on saidreceptacle to about 40 microns in depth.
 5. Method according to claim 1,wherein the boron metal is coated on said receptacle at a temperature ofabout 950* C.
 6. Method according to claim 1, wherein said receptacleconsists of stainless steel.
 7. Method according to claim 1, whereinsaid receptacle consists of cast iron.
 8. Method according to claim 1,wherein said coating of boron metal is effected by electrolysis. 9.Method according to claim 8, wherein said electrolysis is carried out inA molten salt bath comprising an alkali metal borate.
 10. Methodaccording to claim 9, wherein said metal borate is sodium borate. 11.Method according to claim 10, wherein the receptacle is prepared bydipping in a molten salt bath consisting of (i) about 90 percent byweight of borax and (ii) about 10 percent by weight of zirconium oxide,maintaining the bath at a temperature of about 950* C., electrolyzingthe bath for about two hours at a current density of about 0.5 A/cm2,the receptacle being the cathode.